West German Patent Specification No. DE-PS 2,028,027 discloses a sewing machine on whose housing a guide groove is provided in close proximity to the shuttle, and a cutting device is arranged at the end of the guide groove facing away from the shuttle. The guide groove serves to receive the end of the old shuttle thread coming from the fabric being sewn and, after changing the bobbin, to receive the beginning of the new shuttle thread. After the shuttle threads extending in the guide groove have been shortened by the cutting device, the sewing process is continued, and the new shuttle thread is knotted with the needle thread during the first stitching and pulled to the stitch formation site, while the old shuttle thread is gradually pulled out of the guide groove by the feed motion of the fabric being sewn.
Since the needle thread in the sewing machine is not cut when the shuttle thread has come to its end, a seam whose top side is made without interruption is obtained. However, since the needle thread is connected only to the old shuttle thread at the time of the last stitch prior to the change of the bobbin and only to the new shuttle thread at the time of the first stitch after bobbin change, the lower side of the seam has an interruption, which is a weak point under stress.